The invention relates to superconducting magnetic coils.
An important property of a superconductor is the disappearance of its electrical resistance when it is cooled below a critical temperature T.sub.c. Below T.sub.c and for a given superconductor, there exists a maximum amount of current--referred to as the critical current (I.sub.c) of the superconductor--which can be carried by the superconductor at a specified magnetic field and temperature. Any current in excess of I.sub.c causes the onset of resistance in the superconductor. If the superconductor is embedded in or co-wound with a conductive matrix, any incremental current above I.sub.c will be shared between the superconductor and matrix material based on the onset of resistance in the superconductor.
Superconducting materials are generally classified as either low or high temperature superconductors. High temperature superconductors (HTS), such as those made from ceramic or metallic oxides are typically anisotropic, meaning that they generally conduct better, relative to the crystalline structure, in one direction than another. Moreover, it has been observed that, due to this anisotropic characteristic, the critical current varies as a function of the orientation of the magnetic field with respect to the crystallographic axes of the superconducting material. Anisotropic high temperature superconductors include, but are not limited to, the family of Cu--O-based ceramic superconductors, such as members of the rare-earth-copper-oxide family (YBCO), the thallium-barium-calcium-copper-oxide family (TBCCO), the mercury-barium-calcium-copper-oxide family (HgBCCO), and the bismuth strontium calcium copper oxide family (BSCCO). These compounds may be doped with stoichiometric amounts of lead or other materials to improve properties (e.g., (Bi,Pb) .sub.2 Sr.sub.2 Ca.sub.2 Cu.sub.3 O.sub.10).
Anisotropic high temperature superconductors are often fabricated in the form of a superconducting tape having a relatively high aspect ratio (i.e., width greater than the thickness). The thin tape is fabricated as a multi-filament composite superconductor including individual superconducting filaments which extend substantially the length of the multi-filament composite conductor and are surrounded by a matrix-forming material (e.g., silver). The ratio of superconducting material to matrix-forming material is known as the "fill factor" and is generally less than 50%. Although the matrix forming material conducts electricity, it is not superconducting. Together, the superconducting filaments and the matrix-forming material form the multi-filament composite conductor.
High temperature superconductors may be used to fabricate superconducting magnetic coils such as solenoids, racetrack magnets, multiple magnets, etc., in which the superconductor is wound into the shape of a coil. When the temperature of the coil is sufficiently low that the HTS conductor can exist in a superconducting state, the current carrying capacity as well as the magnitude of the magnetic field generated by the coil is significantly increased.
High temperature superconductors have been utilized as current limiting devices to limit the flow of excessive current in electrical systems caused by, for example, short circuits, lightning strikes, or common power fluctuations. HTS current limiting devices may have a variety of different configurations including resistive and inductive type current limiters.